This invention relates to multi-layer, heat-sealable polypropylene films which possess good slip properties and good adhesion to water based adhesives such as water based acrylics, urethanes and vinylidene chloride polymer (PVDC).
Polypropylene films possess a number of desirable characteristics including excellent optical properties such as transparency and brilliance, satisfactory mechanical properties such as tensile strength and Young's modulus, and substantial non-toxic and odorless properties. Accordingly, polypropylene films are widely used as packaging materials, especially for foods. One drawback of polypropylene films, however, is that they possess poor heat sealability. To remedy this, it is widely known to laminate a low-temperature heat-sealable resin to one or both sides of the polypropylene film by coating, laminating or coextruding. Such heat-sealable resins include, for example, middle and low density polyethylenes, ethylene-propylene copolymer, and terpolymers of ethylene, butene and propylene.
Another drawback of unmodified polypropylene films is that they exhibit relatively poor slip characterics, i.e., they exhibit high film to film coefficients of friction which makes it difficult to utilize them in automatic packaging equipment. The poor slip behavior of a film will interfere with its use in automatic processing equipment since the film must pass freely through the fabricating machine (e.g., heat sealer, bag maker, bag loader or filler, bag opener, overwrap package) for it to operate properly and reproducibly. In order to overcome the slip problems in heat-sealable films, it is common to incorporate one or more of several conventional slip additives, e.g., oleamide, stearic acid, erucamide and the like, in the heat-sealable film.
Although crystalline polypropylene films exhibit relatively low vapor permeability, it is often desired to further increase their gas and vapor barrier properties, especially for applications in which the films are being used to package products such as food items which are sensitive to, or attacked by, oxygen or moisture. It is well-recognized in the art that an effective means for increasing the gas and vapor barrier properties of oriented polypropylene films is to coat such films with polymers of vinylidene chloride.
It is important, when coating polypropylene films with such vinylidene chloride polymer compositions, to insure that the adhesion of the coating layer to the polypropylene substrate is adequate. For example, in many packaging applications, it is necessary for the coated film to be heat sealed either to itself or to other films to form a tightly closed package. If the coating adhesion to the base film is inadequate, the packages are liable to prematurely open when subjected to stress.
It has been the common understanding in the art that, to attain adequate adhesion between polypropylene film surfaces and water based adhesives, the film surfaces must be subjected to well known pretreatment operations such as, for example, treatment by corona discharge, flame or oxidizing chemicals. Other widely practiced means for improving the adhesion of the water based adhesives are the coating of the polypropylene film surface with specific primers or the co-lamination of the polypropylene film with an adhesion promotion film. Both coating or colamination methods, however, entail additional processing steps which increase the manufacturing costs of the films.
Even with pretreatment of the polypropylene film, such as by corona discharge, the adhesion of the water based adhesives to the polypropylene film surface will often not be satisfactory when the polypropylene film contains slip additives such as erucamide. Such slip additives tend to bloom or migrate to the surface of the polypropylene film where they act to greatly increase the variability of the bonds between the film and the water based adhesives.
There is therefore a need for a polypropylene films which can run well in packaging machines and which can also form good bonds with water based adhesives. Therefore, a film which exhibits the combined properties of low coefficient of friction and good adhesion to water based adhesives and which can be made in a single manufacturing step is greatly desired.